The present invention relates to high pressure turbines, such as those used in airplane engines, and in which the clearance between the tips of the rotor blades and the stator shroud is controlled by means of a flow of air following at a rate that is controlled.
FIG. 1 shows a high pressure turbine 100 of a turbomachine comprising mainly a plurality of moving blades 102 disposed in a stream 105 of hot gas coming from the combustion chamber (not shown). The moving blades 102 of the turbine are surrounded by a shroud or ring 106. The shroud 106 is secured to a casing 108 of the turbine via a plurality of spacer sectors 110.
In order to increase the efficiency of such a turbine, it is known that the clearance j that exists between the tips of the moving blades 102 of the turbine rotor and the shroud 106 must be reduced to as little as possible.
For this purpose, a device 112 for controlling the clearance j is mounted around the casing 108 of the turbine. This control device 112 comprises in particular annular air-flow manifolds 114 which serve to discharge air onto annular fins 116 of the casing 108 in order to modify the temperature thereof.
The control device 112 is fed with air taken from other portions of the turbomachine (fan or high pressure compressor stage(s)). The flow of air injected into the device 112 is itself controlled by a valve 120 disposed upstream from the device 112. The valve 120 is fitted with an actuator controlled by a control signal Scom which positions the valve in an arbitrary position lying in the range 0% to 100% of its opening so as to determine the flow Fi of air that is injected into the device 112.
The control signal Scom is calculated by a regulator loop which compares the existing clearance j between the tips of the rotor blades and the shroud with a setpoint value corresponding to a predetermined clearance value that is to be reached or maintained. The flow of air Fi injected into the device 112 is then adjusted as a function of the received control signal Scom, thereby enabling the annular fins 116 of the casing 108 to be expanded or contracted thermally so as to vary the diameter of the shroud 106 of the turbine in such a manner as to bring the clearance j to the setpoint value.
However, as explained above, the regulator valve is controlled solely with respect to a single type of setpoint value, i.e. a value representative of predetermined clearance between the tips of the rotor blades and the shroud. This is explained in particular by the fact that this control logic is implemented during the cruising stage of a flight, i.e. the stage that represents the major fraction of flying time. Although controlling clearance enables engine efficiency to be improved, and consequently enables fuel consumption under cruising conditions to be improved, this criterion is not necessarily of greatest priority during other stages of operation of the engine, for example while idling or while taking off, for which potential fuel savings are negligible or for which actually implementing the desired control is not possible (e.g. clearance too great at low temperature while idling).
Thus, with present-day systems, it is not possible to control the flow rate of the air on the basis of setpoint values other than that corresponding to the clearance between the tips of the blades and the turbine shroud.